media: stm32-dcmi: add support of several sub-devices

Add support of several sub-devices within pipeline instead
of a single one.
This allows to support a CSI-2 camera sensor connected
through a CSI-2 to parallel bridge.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>